1. Field of the Invention
Aspects of the present invention relate to an electrode assembly and a secondary battery using the same, and more particularly, to an electrode assembly that includes a ceramic layer disposed adjacent to an outer surface of the electrode assembly, and a secondary battery using the same.
2. Description of the Related Art
Generally, a lithium secondary battery can be rechargeable, and thus, repeatedly usable, which is different from a primary battery, which cannot be recharged. Secondary batteries are widely used as main power sources in high-technology electronic devices, such as, a personal digital assistant (PDA), a notebook computer, and the like. Currently, interest in secondary batteries is increasing and the development of secondary batteries is also being rapidly performed. This is because secondary batteries are generally lightweight, have high energy densities, high output voltages, low discharge rates, and long lifespans.
Secondary batteries are classified into nickel-metal hydride (Ni-MH) batteries, lithium ion (Li-ion) batteries, and the like, based on electrode active materials used therein. The lithium ion (Li-ion) batteries may be classified based on the type of electrolyte, for example, a liquid electrolyte, a solid electrolyte, or a gel-type electrolyte. Also, the secondary batteries are classified into various types, such as, a can type, a pouch type, and the like, based on the shape of a can within which the electrode assembly is disposed.
A lithium ion (Li-ion) battery has an operational voltage of about 3.6V, and is three times more compact than a Ni—Cd battery, or a Ni-MH battery. Also, since the weight-to-energy density of a lithium-ion battery is high, lithium-ion batteries are being rapidly developed. Lithium-ion batteries do not use heavy metals, such as cadmium (Cd) and mercury (Hg), and thus, are more environmentally friendly. A lithium ion battery can be recharged at least one thousand times, in a normal state. Accordingly, with the development in information communication technologies, research into secondary batteries is rapidly occurring, based on the above-described advantages.
Generally, a lithium ion battery (hereinafter referred to as “secondary battery”), includes a can, an electrode assembly, and a cap assembly. The can is in a hollow structure that includes an opening on one side. The opening of the can is sealed by the cap assembly. In other words, the can and the cap assembly form an external case of the secondary battery. The electrode assembly is contained within the can, the opening of the can is sealed by the cap assembly, electrolyte is injected in the can through an opening in the cap, and then the opening is sealed.
The rechargeable electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate. The components of the electrode assembly are wound together. The positive electrode plate includes a positive electrode collector made of a metal foil having an excellent conductivity, for example, aluminum (Al) foil, and a positive electrode active material is coated on both surfaces of the positive electrode collector. Positive electrode non-coating portions, where the positive electrode active material is not coated, are formed on both ends of the positive electrode plate.
The negative electrode plate includes a negative electrode collector made of a conductive metal foil, for example, a copper (Cu) or nickel (Ni) foil, and a negative electrode active material coated on both surfaces of the negative electrode collector. Negative electrode non-coating portions, where the negative electrode active material is not coated, are formed on both ends of the negative electrode plate. Electrode taps are attached to each of the positive electrode non-coating portion and the negative electrode non-coating portion.
As described above, a basic function of the separator is to separate the positive electrode plate from the negative electrode plate, and thereby prevent a short-circuit between the two plates. It is important that the separator absorbs an electrolyte needed for a battery reaction, and has high ion conductivity. In particular, in the case of the lithium ion battery, there is a need to prevent the movement of materials that obstruct the battery reaction, and/or protect against the occurrence of abnormalities. The separator generally includes a polyethylene-based, porous, polymer film, such as, polypropylene, polyethylene, and the like, in one or more layers.
However, the porous film of existing separators is formed as a sheet or film. Accordingly, there are disadvantages that pores of the porous film are blocked, due to generated heat, which can be caused by an internal short-circuit, or overcharging, which cause the sheet-typed separator to contract. When the sheet-typed separator contracts, due to the generated heat, the positive electrode plate contacts the negative electrode plate, where the contraction occurs, and thereby causing the internal short-circuit. Such a short circuit can result in a fire or an explosion.
The film-typed separator may shut-down a secondary battery, by inhibiting the movement of lithium ions, that is, by preventing the flow of current, by softening the polypropylene or polyethylene resin, when the short-circuit and the heat generation occurs. However, the film-typed separator has a fragile structure. For example, in a nail test, which is a simulated internal short-circuit, the temperature at the internal short-circuit locally exceeds millions of ° C., and thus, the transformation of the porous film may be accompanied by the softening or loss of resin, which allows the nail to penetrate the positive electrode and the negative electrode, thereby causing abnormal overheating. Accordingly, the shut-down effect of a resin may not completely prevent the internal short-circuit.
Specifically, as it is required to stably prevent the internal short-circuit between the electrodes, even at a high temperature, a separator including a ceramic layer is provided. The ceramic layer includes a porous film which is formed by coupling ceramic particles with a binder. The separator may be referred to as a ceramic separator. The ceramic layer may be used alone, or along with an existing resin separator.
The ceramic layer of the ceramic separator may be coated on a plate of the electrode assembly. If an internal short-circuit of the battery occurs, the ceramic layer does not contract or melt. Also, the ceramic layer has good charging and discharging properties, and high efficiency, due to having a high porosity. Since the ceramic layer quickly absorbs an electrolyte, an injection speed of the electrolyte is improved.
However, the material cost may increase upon forming of a ceramic layer. Also, the ceramic layer is generally added to an existing resin separator, and thus, the entire volume of a secondary battery may increase, and the mass-to-battery capacity of the secondary battery may be reduced.